This page illustrates how I measured the transmit frequency of a gunnplexer
module with just a wavemeter and another gunnplexer module.

Disassemble one gunnplexer module from its mount and remove the
feedhorn. Attach this module to either one of the wavemeter's two waveguide
ports. Waveguide assemblies and ports only match up in one direction; if you
find that it is difficult to line up the screw holes to attach the gunnplexer
module to the wavemeter port then you have it aligned in the incorrect
direction. If it is not aligned in the correct direction, just rotate it 90
degrees and try again.